Method of making a photoelectric cell



Feb. 6, 1940. Q W, HEWLETT 2,189,580

METHOD OF MAKING A PHOTOELECTRIC CELL Filed May 29, 1937 InventorClarence VV. Hewlett,

is Attorney Patented Feb. 6, 1940- g aisance METHOD F MAKING APHUTOELECTRIC Clarence W. Hewlett, Schenectady, N. Y., assigner toGeneral Electric Company, a corporation of New York Application May 29,1937, Serial No. 145,539

4 Claims. (CLES-8.9)

The present invention relates to light-sensitive devices, moreparticularly to photoelectric cells, which generate a measurableelectromotive force when subjected to light of practical intensities.

5 In my application Serial No. 716,677, filed March 21, 1934, I havedisclosed and claimed a photo-voltaic cell which consists of a metalplate or base member coated first with selenium and then with cadmiumand a relatively non-oxidizable metal such as platinum in order. Thecadmium and platinum layers are preferably laid down bysputtering andare so t-hin as to be semitransparent to light. When light shines onWthe double metal layer, an electromotive force is generated betweenthis layer and the selenium, and the current produced may be conductedaway from the cell by conductors secured in any suitable manner to thissexnitransparent metal layer and to the base member.' This current is ofsuiilcient intensity to be measured by a microammeter which may becalibrated in foot candles to indicate the intensity of the lightimpinging on the metal. The present invention concerns devices of thischaracter, and the objects are to increase the sensitiveness ofphoto-voltaic cells and to lmprove their manufacture. Another object isto Aprovide a practical method of making a rigid contact between one ofthe current-carrying conductors and the thin metal layer. In accordancewith these objects, the sensitiveness of the cell is increased bycontrolling the vapor content of the gaseous chamber in which thesputtering takes place. In order to provide the rigid contact betweenthe conductor and the thin metal 35 layer, a metal ring of vsubstantialthickness is integrally. joined, for example, by spraying, to theselenium surface and the sputtered lm overlays the ring and the seleniumsurface. Then the conductor is pressed in any suitable manner A againstthis ring. The objects andthe manner in which they are attained will bemore clearly understood when reference is made to'the followingspecification and the accompanying drawing in which Fig. l is anelevational, view of a sputter- 45 ing chamber for depositing metallayers on the selenium; Fig. 2 is' a sectional view of a cell in theprocess of making and the fixture for holding the same; Fig. 3 showsanother step in the process; Fig.` i shows an improved method of apply-50 ing a contactring to the cell; and Fig. 5 is a sectional view of thecompleted cell, ready for mounting.

Referring to Fig. 5, numeral I designates a at metal plate of anyconfiguration, for example, circular, and made preferably of iron ornickel. One side of the plate, the upper as shown, is provided with anannular groove 2 and on top of the plate, filling this groove, is alayer of selenium 3. As is pointed out in the said application referredto hereinbefore, there are two metal layers deposited .on the selenium.The first, which is designated by reference character 4, is preferablyof cadmium, and the uppermost metal layer 6 is a relativelynon-oxidizable metal preferably platinum. A contact ring 6 of anysuitable metal such as cadmium is applied in any convenient mannereither to the selenium layer 3 before the sputtered layers have beendeposited, or to the platinum 5 after both metal layers have beendeposited. In case the ring is applied to the selenium layer, thesputtered layers are deposited not only over the selenium layer but alsoover the cadmium ring. The contact ring and the manner of forming thesame constitute one of the features of the present invention and will bedescribed presently. The groove 2 is preferably positioned directlyunder the ring 5 and is slightly wider than the ring as is explained inmy copending application Serial No. 145,538, entitled Photoelectriccells. The purpose of this groove as pointed out in the said applicationis to provide an excess of selenium and thereby increase the resistanceof the path of the current which iiows from the sputtered layer back tothose portions of the selenium from which it started under the iniiuenceof the light. The selenium contained in the groove also serves toprevent a striking-through of the selenium layer when the contact ringis deposited in the manner described hereinafter. Y

In the manufacture of these-cells, the first step is to provide themetal plate I with the annular groove or channel referred tohereinbefore. The side of the plate which is grooved is then roughenedby sandblast in order to provide a good gripping surface for theselenium which is to be deposited. The plate is then heated and wettedwith selenium, the latter being thoroughly rubbed into the surface. Theselenium may be scraped off the plate or disk, leaving the channel fulland only a thin layer of selenium on the surface. The plate is thenchilled. This thin wetting layer of selenium is then recrystallized byheating the plate. The plate with the grooved side up is then placed ina heated heavy metal xture 8, shown in Fig. 2, which has a countersunkor depressed portion of a size snugly to receive the plate. This,countersunk portion is of a depth greater than the thickness of theplate i by an amount representing the thickness of the selenium layerwhich is to be deposited on the plate. Selenium, in paste, form, isspread on the disk l and the excess is scraped od by a tool 9 whichbridges the countersunk portion of the xture t and leaves a layer ofselenium of the proper thickness depending upon the depth of the recessin the fixture. After a smooth surface has been obtained on the seleniumby drawing the scraper in several directions'across the upper surface ofthe fixture 8,

the fixture and the element is chilled. The next step in the process isto obtain a smooth, glassy appearing, compact layer of selenium ofuniform thickness on the plate, and this is conveniently accomplished bya hot press treatment.

A suitable fixture for this treatment is shown in Fig. 3 and consists ofa metal plate P containing heating elements (not shown) let into holesH, and a die I containing a recess of a depth corresponding to thenished thickness of the selenium coated plate. Before placing theselenium plate on the press, the apparatus is heated to a temperature ofabout 140 C., and the selenium coated plate I is then placed, seleniumside down, against the plate P. The die I0 is then placed on top of theselenium coated plate, and pressure applied in any suitable manner, forexample by means of a clamp. At the temperature mentioned, the seleniumsoftens and flows, the excess being exuded around the edges of the plateas indicated at B. Although as stated hereinbefore, the selenium at rstsoftens and flows, in a few minutes it begins to harden, and after 10 to30 minutes the pressure may be relieved and the plate removed fromfixture. The selenium layer is now hard, smooth, and of uniformthickness, and the excess exuded selenium may be removed from the edges.The selenium coated plate I which has thus been treated is then baked ata temperature near the melting point of selenium.

The plate I is then placed in a xture similar to that shown in Fig. 4for the deposition of the contact ring. This fixture comprises a heavymetal ring 36 which has a metal back 31 secured thereto by s'crews 38.The front or upper portion of the ring 36 is provided with an inwardlyextending lip 38 against which the selenium coated disk I can be pressedby a spring 40 riveted to the back plate 31. A conically shaped metalshield 4I provided with a handle 42 is provided for protecting thecentral portion of the metal or selenium coated disk from the heateffects of a Schoop gun 45. The shield 4I has an outer diameter lessthan the lip.39 so as to leave an annular space. A heavy metal layer ofcadmium is laid down in this space by the gun 45, in the form of a ring.As stated hereinbefore, this ring is positioned directly above theselenium-filled groove 2 formed in the plate I and is slightly less inwidth than the width of the channel. This contact ring is for thepurpose of readily permitting a connection between a conductor (notshownl and the metal layer which is to be sputtered over the seleniumsurface, as Will be described presently, and lapping over the contactring. After the contact ring has been built up to its proper thickness,the cell is removed from the fixture by unscrewing the plate 31, and itis then ready to be sputtered with the metal layers. While I prefer toapply the contact ring ,directly to the selenium surface, I may, undercertain circumstances, deposit the ring on the sputtered metal layer.

The sputtering apparatus is shown in Fig. 1 and consists essentially ofa large bell jar I2 which terminates at the top in a hemisphericalportion I3. A reentrant bulb I4 having an opening I5 is secured to thehemispherical portion I3. The lower end of the bell jar is temporarilyclosed in any suitable manner, for example, by being inserted within aclose-tting recessed plate member I6 which is well known in the art.Secured to the plate member I6 and centrally'positioned, there is ametal upright or standard I1 which carries a rotatable drum I8 and astationary rod member I9. A flat metal plate 20 is secured to the upperend of the rod I9. 'I'he purpose of this plate will be explainedpresently. 'I'he drum member I8 carries a pair of oppositely extendingshafts 2|, 22, one of which (element 2I) terminates in a counterweight23. The other shaft 22 carries a magnetic member 24 of arcuate form, andat a position intermediate the ends of the rod, also carries an uprightshaft 25. A carrier fixture 26 provided with a central recess 21 issecured to the upper end of the rod 25. This recess has a diametral sizeand thickness sufiicient to accommodate the selenium coated disk I. Inaddition to the upright I1, the base I6 is also provided with a glassupright 28 which terminates at the top in a pair of forked arms 26. Thearms 29 contain a pair of conductors 3D, 3|, one of which is secured toa mesh member 32 composed of platinum, and the other carries a disk 33of cadmium which lies directly above the carrier 26.

The arrangement is such that by means of a magnet 34, which is adaptedto be moved around the bell jar, the armature 24 is caused to movewithin the chamber and this movement serves to rotate the carrier 26from a position directly under the disk 33, as shown in Fig.` 1, to .aposition directly under the mesh member 32. When it is desired todeposit a metal layer on the selenium-coated disk I, the bell jar istemporarily removed and the disk I is placed in the recess of thecarrier 26. A mask (not shown) is placed about the periphery of thecell, the purpose of which is to shield the edges of the cell from thesputtered deposit. This mask allows the sputtered deposit to cover thecentral portion of the selenium surface and partly to overlap thesprayed-on cadmium ring. After the bell jar is placed in position andproperly sealed, the jar is evacuated in any suitable manner. Argon orany other inert gas is introduced into the bell jar at about 240microns. By means of the leads 30 and 3| which pass to the exterior,either of the electrodes 32 and 33 may be made cathode of a circuit (notshown), and by means of a lead (not shown) the plate I6 may be made theanode. The selenium plate is then sputtered under the cadmium cathode inone-second flashes separated by 20-second intervals until a sputteredlayer which permits about 30% of the incident light to pass through isobtained. The selenium plate is then adjusted under the platinum cathodeby moving the magnet 34 around the bell jar, and sputtered with platinumfor about three-fifths the length of the time of sputtering the cadmium.It is apparent that the metal plate 20 serves as a partition to preventsputtered deposits from one cathode being deposited on the othercathode.

During the sputtering operation, liquid air may be poured into the askI4, causing condensible,

vapors such as water to condense on the surface of the flask, therebyremoving them from the gaseous atmosphere. While satisfactory resultshave been obtained by causing the condensation to take placesimultaneously with the start of the sputtering operation and evenbefore, I have found, and in accordance with the present invention, thatoutstandingly good cells may be obtained if the introduction of liquidair and the condensationare delayed until an appreciable time after thesputtering of the cadmium layer has started. I prefer to delay theintroduction ofthe liquid air until from one to five flashes of thecadmium sputtering have been carried out. After the liquid air has beenintroduced in this manner and the cadmium sputtering has been continueduntil the proper. thickness of layer has been obtained, the platinumlayer is sputtered in the usual manner. as explained above. It has beenfound that when the refrigeration of the gaseous atmosphere has beendelayed for a short time and is not produced simultaneously with the "ibeginning of the cadmium sputtering. the resulting cell is moresatisfactory than those cells in which the refrigeration is initiatedsimultaneously with the sputtering. The improved cell will delivermorepower in an external circuit of higher resistance, and the temperaturecoeiiicient of output and the fatigue under action of illumination areboth considerably smaller. As a matter of fact, a cell so produced willhave approximately double the open circuit electromotive force of one inwhich the refrigeration has not been delayed.

Afterthesputtering operations described above, the plate I is removedfrom the carrier 28 and the selenium surface up to the contact ring islacquered or varnished as indicated at 48, Fig. 5. The cell is nowcomplete and laid aside to mature or age.

While I do not wish to be limited to any theory as to why such a markedimprovement is brought about by simply delaying the application of therefrigerant, I believe that the action might be as follows. It is ofcourse well known that the so-called "blocking layer of a cell of thischaracter and its ability to generate electromotive force when exposedto light depend on the character of the material at the junction betweenthe selenium surface and the semitransparent 'layers of cadmium andplatinum. In my improved the selenium surface.

cell, this intervening layer of material is probably composed ofselenium dioxide, cadmium oxide, or a mixture of these two oxides withcadmium. There are several methods by which this end can be obtained: 1)By exposing the. selenium surface in a glow discharge of oxygen, inwhich case the selenium surface may or may not be made cathode of thedischarge, (2) by evaporating selenium dioxide and cadmium 'oxide ontothe selenium surface (in this case some decomposition of cadmium oxidemay occur so that the resulting deposit includes a mixture of cadmiumand cadmium oxide) (3) by the preferred method of dampening the cadmiumelectrode before exhausting the sputtering apparatus and then sputteringfor a short time before refrigeration (by liquid air or other suitablerefrigerant placed in the reentrant flask I4) in the sputtering space.This preliminary sputtering using the cadmium electrode as cathodeliberates the residual'water from the cadmium cathode, and the watervapor becomes ionized in the discharge. The resulting oxygen ionsioxidize the selenium and the cadmium, resulting in a layer of seleniumdioxide and sputtered cadmium oxide and cadmium on The -sputtering spaceis then refrigerated to remove vapors from the sputtering discharge andthe cadmium vsputtering proceeds, after which the platinum layer isproduced. Regardless of whether this theory is correct or not, the factremains that when foreign substances such as water vapor are permittedto remain in the argon atmosphere during the initial stages of thesputtering process, some action takes place which causes the seleniumsurface to be more light-sensitive or causes a more eifective barrierlayer between the selenium surface and the cadmium to be formedso that acell in which the refrigeration has been delayedfor a predeterminedlength of time is much more sensitive than a cell in which therefrigeration has been provided simultaneously with the initialsputtering stages.

As was pointed out in my application Serial No. 716,677, a cell of thiskind may be inserted in a casing of hard rubber and when connectionshave been made to the contact ring and the metal base I, the cell willgenerate an electromotive force upon exposure to light which issufiicient to operate a microammeter. 'I'his meter may be calibrated infoot candles to indicate or measure the intensity of light falling onthe metal layers. i0

What I-claim as new and desire to secure by Letters Patent of the UnitedStates, is: 1. In the method of fabricating a light-sensitive device,the steps of coating a metal plate with selenium, then sputtering lasemitransparent lay- 28 er of cadmium onto said selenium in a chambercontaining argon and water vapor, then condensing out the water vapor bycooling said chamber, then sputtering an additional semitransparentlayer of cadmium onto the iirst layer-` of cadmium, and finallysputtering a semitransparent layer of platinum onto the cadmium.

2. In the method of fabricating a selenium cell the steps of coating ametal plate with a layer of selenium, then sputtering a semitransparentlayer of cadmium onto said selenium from a dampened cadmium surface andthen sputtering a semitransparent layer of platinum onto the cadmiumlayer.

3. In the method of fabricating a light-sensitive device, the steps ofcoating a base member with selenium, positioning the base member in aclosed chamber in proximity to a source of cadmium. evacuating thechamber, introducing a quantity of an inert gas into the chamber,producing an electrical discharge within the chamber to cause sputteringof a thin layer of cadmium on the selenium, cooling a portionof thechamber to a temperature comparable with that of liquid air, thereby tovremove any concause sputtering of a thin layer of cadmium on theselenium, cooling a portion of the chamber to a temperature comparablewith that of liquid air, thereby to condense any condcnsibleconstituents contained in the chamber, thereafter sputtering a furtherlayer of cadmium on the first layer thereof, and nally depositing athinlayer of a relatively non-oxidizable metal on the cadmium.

CLARENCE W. HEWLETT.

